目的 探明超疏水-疏水相间表面气-液界面形态随时间的动态演化过程,揭示超疏水-疏水相间表面减阻特性随时间、流速的演变规律。方法 在7075铝片上,通过调控紫外纳秒激光的脉冲能量、扫描速度和加工路径,制备封闭微腔阵列结构,然后预留特定宽度的带状区域不进行激光加工,得到微腔阵列结构和未加工区域相间的表面,再经过低表面能物质（氟硅烷）处理,得到超疏水-疏水相间表面,其中激光加工区呈超疏水性（接触角为158.8°、滚动角为0.8°）,未加工区呈疏水性（接触角为131.2°）。结果 流速为1.53~4.43 m/s时,超疏水-疏水相间表面的超疏水区会形成气膜,并附着长条状的气泡带（简称“气泡条”）,具有减阻效果。在低流速条件下（1.53 m/s）,当疏水区的宽度为0.5 mm时,表面最高减阻率为35.56%,优于均匀超疏水表面的减阻率（32.86%）。随着水流作用时间不断增加,气泡条和气膜的覆盖面积均逐渐减小至消失,气泡条的维持时长约12 min,气膜的维持时长可达6 h,在0.5 h内超疏水-疏水相间表面的平均减阻率高于均匀超疏水表面。在高流速条件下（4.43 m/s）,超疏水-疏水相间表面的减阻率低于均匀超疏水表面。结论 在较低流速和短时间内,由于气泡条的存在使超疏水-疏水相间表面的减阻效果优于均匀超疏水表面。该研究对于提高水下航行器减阻率、发展可持续减阻策略、满足多样化应用领域的减阻需求具有重要借鉴意义。
Abstract
Superhydrophobic surfaces have great potential in reducing the frictional drag of underwater vehicles. Uniform superhydrophobic surfaces can trap air underwater and form an air layer, but the stability and morphologies of liquid-air interfaces on superhydrophobic-hydrophobic surfaces and the corresponding drag reduction performance still remains unknown. This work aims to fabricate superhydrophobic-hydrophobic surfaces to investigate the stability of the liquid-air interfaces and examine the drag reduction performance. Closed microcavities were fabricated on the surface of 20 mm × 80 mm × 5 mm 7075 aluminum alloy plates by adjusting the pulse energy, scanning speed and path of a violet nanosecond laser. The specific areas of different widths (0.5, 1.0, 1.5, 2.0, 2.5 mm) were reserved to avoid being fabricated by laser, by which different types of superhydrophobic-hydrophobic surfaces were obtained after fluorination treatment. The contact angles of the superhydrophobic region and the hydrophobic region were 158.8° and 131.2°, respectively. The sliding angle of the superhydrophobic region was 0.6°. Air layer observation and drag reduction evaluation were performed with a home-built circulating water tunnel facility, which included a water tank, a circulating pipe, a pump, a test section, and two valves. This device could simulate underwater navigation conditions by using flowing liquid that completely immersed the sample to be tested. When the flow rate was in the range of 1.53-4.43 m/s, both air layers and air bubbles could be formed on the superhydrophobic-hydrophobic surface, whereas only an air layer could be formed on the uniform superhydrophobic surface. The air bubbles existed in the form of strips, namely "air bubble strips", which were located at the junction of the superhydrophobic region and the hydrophobic region. These air bubble strips had a significant drag reduction effect under certain flowing conditions. At the lower flow rate (1.53 m/s), the maximum drag reduction rate was 35.56% when the width of hydrophobic region was 0.5 mm, which was higher than that of the uniform superhydrophobic surface. With the increase of water flowing time, the coverage of air bubbles and air layers gradually decreased and finally disappeared. The air bubbles could sustainably maintain for ~12 min whereas the air layer could maintain for ~6 h. The average drag reduction rate of the superhydrophobic-hydrophobic surface was higher than that of the uniform superhydrophobic surface within 0.5 h. At the higher flow rate (4.43 m/s), the average drag reduction rate of the superhydrophobic-hydrophobic surface was lower than that of the uniform superhydrophobic surface. As a result, the drag reduction effect of the superhydrophobic-hydrophobic surface was better than the uniform superhydrophobic surface within 0.5 h when the flow rate was relatively low. Hence, at lower flow velocities and within a short period of time, due to the bubble strip, the drag reduction effect of the superhydrophobic-hydrophobic interfacial surface was superior to that of the uniform superhydrophobic surface. This study can provide significant guidance for enhancing the drag reduction rate of underwater vehicles, developing sustainable drag reduction strategies, and meeting the drag reduction requirements in their diverse application fields.
关键词
减阻 /
超疏水-疏水相间表面 /
超疏水表面 /
气泡 /
气膜
Key words
drag reduction /
superhydrophobic-hydrophobic surface /
superhydrophobic surface /
air bubble /
air layer
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